Electromagnetically resonant device

ABSTRACT

A self contained electromagnetically resonant device includes a resonant circuit assembly encapsulated in an elastomeric material surrounded by a rigid casing. The resonant circuit includes an elongated core of sintered magnetically soft material and a coil connected across a capacitor which is spaced from the core. The elastomeric filler co-acts with the casing to substantially retain the core in its original form and dimension in the event of core breakage to maintain the resonant frequency and quality of the resonant circuit assembly. The device is resonantly excited solely by exposure to an electromagnetic or magnetic field having a suitable frequency component.

United States Patent Zimmermann et al.

ELECTROMAGNETICALLY RESONANT DEVICE Inventors: Detlef Zimmermann,Ottawa,

Ontario; James Hugh Lougheed, Kanata, Ontario; Eric John Crompton,Montreal West, Quebec, all of Canada Assignee: Northern Electric CompanyLimited,

Montreal, Quebec, Canada Filed: Mar. 7, 1974 Appl. No.: 448,876

US. Cl. 343/18 B; 324/34 R; 324/41 Int. Cl. G015 15/14 Field of Search343/18 B, 6.8 R, 6.5 LC;

Erdmann 324/4l Horchler 273/213 Primary Examiner-Maynard R. WilburAssistant ExaminerG. E. Montone Attorney, Agent, or Firm-John E. Mowle[57] ABSTRACT A self contained electromagnetically resonant deviceincludes a resonant circuit assembly encapsulated in an elastomericmaterial surrounded by a rigid casing. The resonant circuit includes anelongated core of sintered magnetically soft material and a coilconnected across a capacitor which is spaced from the core. Theelastomeric'fi'ller co-acts with the casing to substantially retain thecore in its original form and dimension in the event of core breakage tomaintain the resonant frequency and 'quality of the resonant circuitassembly. The device is resonantly excited solely by exposure to anelectromagnetic or magnetic field having a suitable frequency component.

5 Claims, 6 Drawing Figures ELECTROMAGNETICALLY RESONANT DEVICE Thepresent invention relates to electromagnetically resonant devices,particularly devices having a relatively fragile magnetic core materialwhich is suitably protected to maintain satisfactory resonance operationeven in the event of core breakage.

A self-contained passive resonant circuit is disclosed in co-pendingapplication Ser. No. 325518 filed in the name of D. Zimmermann et al. onJan. 22. 1973 now U.S. Pat. No. 3836842. The resonant circuit includes aferrite core and an inductor connected across a capacitor. The circuitis intended for use in a field environment which dictates that it mustwithstand rough handling without significant degradation or change inits resonant frequency or quality. It was found that rough handlingtended to break or shatter the core material. a sintered magneticallysoft material often referred to as a ferrite. Such breakage usuallyalters the resonant frequency and quality. to such an extent. that thedevice becomes unuseable for the purpose intended. i.e. as anelectromagnetic marker. It was determined that less fragile cores. forexample of sheet laminates or rod bundles, did not have magneticproperties best suited to device operation.

Hence in the above mentioned application. a resonant circuit assemblyencapsulated in a material at least as brittle as the ferrite corematerial is disclosed. This carries the advantage that if the corebecame fractured the encapsulation is likewise fractured and anunserviceable device is so identified by visual inspection. This atleast prevents unserviceable devices from being installed or buried asmarkers which would not. in some cases. resonate adequately to belocated.

This form of encapsulated resonant circuit has various minordisadvantages. Although it is reinforced by the encapsulation. asubstantial fraction of the total are damaged before use. Core breakagein the area of the coil winding may not be detected visually because ofthe reinforcing effect of the wire in the induction coil. In some cases.this reinforcing initially permits only a hair line type of fracture,easily overlooked during a visual inspection.

The present invention provides an encapsulation which, in the event ofcore breakage or shatter. resiliently maintains the core form anddimension. Consequently, resonant frequency and quality of a device issubstantially maintained. As long as the other shell of the deviceremains unbroken, that device will function satisfactorily in the field,as a marker, regardless of the mechanical integrity of the core.

Difficulties have been encountered, in mass producing a coil wound uponan elongated core, as described in the above mentioned co-pendingapplication. The ferrite core material is fragile and not economicallyformable into very straight lengths. Hence typical coil winding machinestend to shatter the matter if winding directly on the core is attempted.If an elongated bobbin is used to carry the desirably elongated coil.the bobbin must be flexible enough to accommodate deviations in theferrite core. Again, bobbins of preferred flexibility are not readilyadaptable to typical mass production coil winding methods.

Some difficulty was experienced in producing resonant devices having asuitable high resonant quality. It was found that the capacitive elementof the resonant circuit if placed next to the core, significantlyreduced the potential quality of resonance of the circuit. To avoidthis, the capacitor is spaced apart from the core.

The present invention is a self contained electromagnetically resonantdevice, excitable solely by exposure to an electromagnetic or magneticfield having a suitable frequency component. The device comprises aresonant circuit assembly including a coil residing on an elongated coreof sintered magnetic material and a capacitor connected across the coiland spaced adjacent the core. The resonant circuit assembly resideswithin a rigid hollow casing and is surrounded by an elastomeric fillercontiguous with the assembly and with the walls of the casing. In theevent of core breakage, the elastomeric filler co-acts with the casingto substantially retain the core in its original form and di' mension,so that the resonant frequency and quality are substantially maintained.

The present invention also includes a relatively short coil carried on abobbin having an interior dimension substantially greater than thethickness of the core and resilient gripping means to maintain therequired position of the coil relative to the core. The bobbin is ro'bust to be compatible with typical mass production coil winding methods.

The present invention also includes a retaining member which carries theresonant circuit assembly of the core and the coil spaced apartsufficiently from a capacitor. to prevent any substantial reduction ofthe quality of resonance of the circuit.

An example embodiment of an electromagnetically resonant circuit deviceis discussed in the following with reference to the accompanyingdrawings in which:-

FIG. 1 is a side elevation of an electromagnetically resonant device inaccordance with the invention;

FIG. 2 is a cross-sectional view taken along a line [H1 in FIG. 1;

FIG. 3 is an exploded assembly view of the device in FIG. 1;

FIG. 4 is a side elevation of one part of the device in FIGS. 1 and 2;

FIG. 5 is a top view of the part in FIG. 3; and

FIG. 6 is a cross-sectional view taken along a line Vl-VI in FIG. 3.

Referring to the drawings a resonant circuit assembly includes aninduction coil 4 and a ferrite core 5 wound on a bobbin 32. The bobbin32 includes a pair of resilient fingers 33 extending from one endthereof. The bobbin 32 is coaxially carried on the core 5. The core 5includes six longitudinal channels 6 formed therein. The channels 6provide a much larger surface area on the core 5 than if the core wereof a plain circular cross-section. The coil 4 is connected via leads 7across a capacitor 8. The core 5, the coil 4 and the capacitor 8constitute an electrically resonant circuit assembly which resideswithin a rigid outer casing 10. The remaining volume in the casing I0 isoccupied by an elas tomeric material 35 and a retaining member 20.

The retaining member 20 includes a hollow projection 21 which engagesthe inner surface of the wall of the casing 10. One end of theprojection 21 terminates at an outwardly projecting grooved shoulder 23.The other end of the projection 21 terminates at an end wall 35 whichcarries a hollow cylindrical projection 25, within which an end of thecore 5 resides. The interior of the hollow projection includes a pair ofresilient retaining fingers 29, extending on one side thereof from theend wall 25 and another resilient retaining finger 28 opposite thefingers 29 and likewise extending from the end wall 25. The end portionsof the retaining fingers 28 and 29 are adapted to grasp and retain thecapacitor 8, which is of tubular form. The hollow projection 21 includesa pair of captive openings 22 to facilitate the placement of thecapacitor 8 and connection of the coil 4 thereto.

The casing 10 includes a large diameter portion 11, terminating atshoulders 16 and 17 which extend into lesser diameter portions 12 and 13respectively. As illustrated in FIG. 3, the end of the lesser diameterportion 13 tapers away from the large diameter portion 11 and terminatesat a blunt end 15. The walls of the casing 10 include a plurality oflongitudinal reinforcing formations 14 to enhance the longitudinalrigidity of the casing 10.

In assembly of the device the electrical components are held by theretaining member 20. The electrical components of the assembly are ofcourse of such value that the circuit will resonate within a narrowpredetermined frequency range. The exact resonant frequency desired isobtained by exciting the circuit to resonance and adjusting the positionof the coil 4 along the length of the core to obtain the exact desiredfrequency of resonance. The fingers 33 provide gripping means to holdthe coil 4 carried by the bobbin 32 at the desired position on the core5 during the remainder of the assembly operation. The casing is filledto a certain level with a cureable liquid resin material. The electricalcomponents and the retaining member 20, as one unit. are inserted intothe casing until the shoulder 23 fits against the end of the portion 12.The interior of the bobbin 32 is of sufficient diameter to permit alayer of the resin to flow between the core 5 and the bobbin 32. Theassembled device can be viewed looking into the hollow projection 21which should now be substantially filled, as is the remainder of thecasing 10, with the resin material. After the resin is cured the deviceis ready for use.

The resin in its cured state provides the elastomeric material 35. Theelastomeric material 35 is by nature substantially incompressable. Inother words the volume of the filler 35 remains substantially constanteven under considerable stress. As before mentioned the elastomericmaterial 35 is contiguous with the casing 10 and the items containedwithin. The channels 6 provide a relatively large surface area on thecore 5 which enhances the retaining action of the encapsulation upon thecore 5. The large diameter portion 11 of the casing 10 provides spacefor an adequate amount of the elastomeric material between the coil andthe wall of the casing 10. The shoulder 16 restrains the material 35from any tendency to work its way through the portion 12 and out of theend of the casing 10.

With the core length between about 6 and 8 inches in length, it has beenfound that the capacitor must be separated from the end of the core byat least onefourth of an inch. This minimizes the velocity and densityof magnetic flux cutting through the plates of the capacitorsufficiently to avoid a significant reduction in the quality of theresonant circuit.

In one embodiment the casing 10, and the retaining member are of a highdensity polyethylene material. The casing 10 is formed by blow moldingin a well known manner. The assembly fixture 20 is also of a highdensity polyethylene material however somewhat rigid than the materialof the casing 10 so that the retaining fingers 28 and 29 have adequateresilience to facilitate the placement and retention of the capacitor 8.The fingers 28 and 29 may be substituted by a raised projection orpedestal upon which the capacitor rests. However this may require someother means for holding the capacitor in position at least until thematerial 35 has cured. The bobbin 32 is of a stabilized polypropylenematerial. The elastomeric material 35 is a polyurethane compound of thenon-adhesive type. For example, a polyurethane material supplied byBaker Castor Oil Co. in Bayonne, N.J., U.S.A. in curable resin form,under the trade name of System 86 or System 27 Polyurethane, has beenfound suitable.

In the event of a severe momentary mechanical shock or stress to thedevice, it is likely that the ferrite core material will break while thecasing 10 will typically only flex. Any tendency for dimensional changeof the core 5 is substantially resisted and prevented by thepolyurethane material 35 retained in the casing 10. Resonant frequencyshifts of typically less than about 4 percent have been experienced, indevices initially resonant at about 65 KHz and in which a majorityportion of the core has become shattered. These damaged devices remaineffectively useful as electromagnetically resonant marking devices.

What is claimed is:

l. A self contained electromagnetically resonant device, excitablesolely by exposure to an electromagnetic or magnetic field having asuitable frequency component, the device comprising:

a rigid hollow casing;

a resonant circuit assembly having, an elongated core of sinteredmagnetically soft material. a coil of conductive material encircling aportion of the core, and a capacitor connected across the coil, thecapacitor being spaced from the core to reduce magnetic flux velocitythrough the plates of the capacitor, the assembly residing within thecasing, spaced adjacent the wall thereof;

an elastomeric filler within the casing, surrounding said assembly,contiguous therewith and with the walls of the casing, the elastomericfiller co-acting with the casing to substantially retain the core in itsoriginal form and dimension, in the event of core breakage, so that theresonant frequency and quality of the resonant circuit assembly issubstantially maintained.

2. A device as defined in claim 1 in which the core includes a pluralityof longitudinal channels, said channels providing a relatively largesurface area contiguous with said elastomeric filler to enhance saidretaining action.

3. A device as defined in claim 1 in which the coil resides on a bobbincarried by the core, the interior of the bobbin being of greaterdimension that the thickness of the core and a layer of the elastomericfiller residing between the core and the interior surface of the bobbin.the bobbin including resilient gripping means for maintaining the coilat a desired position along the core prior to and during final assemblyof the device.

4. A device as defined in claim 1 in which the hollow casing open at oneend and includes a retaining member for holding the elements of theresonant circuit assembly in position during assembly of the device, theretaining member comprising:

a wall defining an interior cavity and mating with the inner surface ofthe casing adjacent the opening;

a shoulder terminating one end of the wall and in abutment with the endof the casing wall;

a retaining cup terminating the other end of the wall. one end of theelongated core residing within the retaining cup;

spacer means protruding into the interior cavity and carrying thecapacitor spaced adjacent the one end of the core;

the wall of the retaining member including a captive opening adjacentthe spacer means. the opening providing for placement of the capacitoradjacent the spacer means and for the flow of the elastomeric materialin its uncured state during insertion of the resonant circuit assemblyinto the casing.

5. A self contained electromagnetically resonant device, excitablesolely by exposure to an electromagnetic or magnetic field having asuitable frequency component, the device comprising:

an elongated rigid hollow casing open at one end and including aplurality of longitudinal reinforcing formations in the wall of thecasing. the end portion of the casing adjacent the open end thereofbeing of lesser cross-sectional area than the middle portion of thecasing. the middle and end portion being joined by a shoulder portion ofthe casing wall;

a resonant circuit assembly having, an elongated core of sinteredmagnetically soft material, a capacitor, and a bobbin carrying aninductor and residing on the core. the inductor electrically connectedacross the capacitor, the bobbin including a pair of spring fingersextending from one end of the bobbin to frictionally grip the core. toretain the inductor thereon;

retaining member carrying the resonant circuit assembly and fitting intothe open end of the casing to maintain the resonant circuit assemblywithin and spaced from the wall of the casing and to maintain thecapacitor spaced adjacent the end of the core during assembly of thedevice;

an elastomeric filler, surrounding said assembly. contiguous therewithand with the walls of the casing, and constrictively retained within thecasing by said shoulder portion of the casing wall. in the event of acore breakage, the elastomeric filler coacting with the casing tosubstantially retain the core in its original form and dimension, sothat the resonant frequency and quality of the resonant circuit assemblyis substantially maintained.

1. A self contained electromagnetically resonant device, excitablesolely by exposure to an electromagnetic or magnetic field having asuitable frequency component, the device comprising: a rigid hollowcasing; a resonant circuit assembly having, an elongated core ofsintered magnetically soft material, a coil of conductive materialencircling a portion of the core, and a capacitor connected across thecoil, the capacitor being spaced from the core to reduce magnetic fluxvelocity through the plates of the capacitor, the assembly residingwithin the casing, spaced adjacent the wall thereof; an elastomericfiller within the casing, surrounding said assembly, contiguoustherewith and with the walls of the casing, the elastomeric fillerco-acting with the casing to substantially retain the core in itsoriginal form and dimension, in the event of core breakage, so that theresonant frequency and quality of the resonant circuit assembly issubstantially maintained.
 2. A device as defined in claim 1 in which thecore includes a plurality of longitudinal channels, said channelsproviding a relatively large surface area contiguous with saidelastomeric filler to enhance said retaining action.
 3. A device asdefined in claim 1 in which the coil resides on a bobbin carried by thecore, the interior of the bobbin being of greater dimension that thethickness of the core and a layer of the elastomeric filler residingbetween the core and the interior surface of the bobbin, the bobbinincluding resilient gripping means for maintaining the coil at a desiredposition along the core prior to and during final assembly of thedevice.
 4. A device as defined in claim 1 in which the hollow casingopen at one end and includes a retaining member for holding the elementsof the resonant circuit assembly in position during assembly of thedevice, the retaining member comprising: a wall defining an interiorcavity and mating with the inner surface of the casing adjacent theopening; a shoulder terminating one end of the wall and in abutment withthe end of the casing wall; a retaining cup terminating the other end ofthe wall, one end of the elongated core residing within the retainingcup; spacer means protruding into the interior cavity and carrying thecapacitor spaced adjacent the one end of the core; the wall of theretaining member including a captive opening adjacent the spacer means,the opening providing for placement of the capacitor adjacent the spacermeans and for the flow of the elastomeric material in its uncured stateduring insertion of the resonant circuit assembly into the casing.
 5. Aself contained electromagnetically resonant device, excitable solely byexposure to an electromagnetic or magnetic field having a suitablefrequency component, the device comprising: an elongated rigid hollowcasing, open at one end and including a plurality of longitudinAlreinforcing formations in the wall of the casing, the end portion of thecasing adjacent the open end thereof being of lesser cross-sectionalarea than the middle portion of the casing, the middle and end portionbeing joined by a shoulder portion of the casing wall; a resonantcircuit assembly having, an elongated core of sintered magnetically softmaterial, a capacitor, and a bobbin carrying an inductor and residing onthe core, the inductor electrically connected across the capacitor, thebobbin including a pair of spring fingers extending from one end of thebobbin to frictionally grip the core, to retain the inductor thereon; aretaining member carrying the resonant circuit assembly and fitting intothe open end of the casing to maintain the resonant circuit assemblywithin and spaced from the wall of the casing and to maintain thecapacitor spaced adjacent the end of the core during assembly of thedevice; an elastomeric filler, surrounding said assembly, contiguoustherewith and with the walls of the casing, and constrictively retainedwithin the casing by said shoulder portion of the casing wall, in theevent of a core breakage, the elastomeric filler co-acting with thecasing to substantially retain the core in its original form anddimension, so that the resonant frequency and quality of the resonantcircuit assembly is substantially maintained.